Power supply regulators, especially in switch mode power supplies, have their outputs regulated by means of a switch, an inductor with a winding, and a reservoir capacitor. The switch allows current to be delivered through the inductor to a reservoir capacitor. The switch is operated repetitively, usually in sympathy with switching in the switch mode power supply. The switch is controlled to be “on” or “off” for a proportion of the time that reflects the deviation experienced by the reservoir capacitor voltage from a target voltage.
The more current drawn from the reservoir capacitor, the greater the proportion of time the switch is closed. When no current is drawn, the switch is open for nearly the entire proportion of the cycle time. The target voltage may be fixed, as for a DC power supply, or may be varied, reflecting the instant AC target voltage for AC power supply situations.
Several problems exist with such a simple arrangement. In order to maintain a small ripple voltage, the current supplied to the reservoir capacitor by collapse of the magnetic field must never cease to flow. This is called continuous operation. In high current demand situations, if the switch operation frequency is too low, the magnetic field of the inductor can become discharged and the reservoir capacitor voltage can display a ripple component that is unacceptably large.
Even with continuous operation, the ripple on the reservoir capacitor can be undesirably large. The present invention further seeks to solve this problem by providing an apparatus operable to reduce ripple to a minimum level and ideally to reduce ripple virtually to zero.
A second problem is that the power drawn from the supply to the switching circuit also may have a large ripple current exacerbated by some switching topologies such as in FIG. 3A requiring no current flow into the input for a significant part of each cycle of operation. This means that the supply to the switching circuit itself may require extra filtering and storage components to smooth out these current variations.
The present invention seeks to provide an apparatus solving the aforementioned problems, and in particular, by lengthening the time over which current is delivered to the reservoir capacitor.